Light-emitting device

ABSTRACT

According to present invention, system on panel without complicating the process of TFT can be realized, and a light-emitting device that can be formed by lower cost than that of the conventional light-emitting device can be provided. A light-emitting device is provided in which a pixel portion is provided with a pixel including a light-emitting element and a TFT for controlling supply of current to the light-emitting element; a TFT included in a drive circuit and a TFT for controlling supply of current to the light-emitting element include a gate electrode, a gate insulating film formed over the gate electrode, a first semiconductor film, which overlaps with the gate electrode via the gate insulating film, a pair of second semiconductor films formed over the first semiconductor film; the pair of second semiconductor films are doped with an impurity to have one conductivity type; and the first semiconductor film is formed by semiamorphous semiconductor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a light-emitting device using a thin filmtransistor for a drive circuit and a pixel portion.

2. Related Art

In a semiconductor display device formed by using a glass substrate atlow cost, the required area for the periphery of a pixel portion (aframe region) for mounting is increased with the increase of screenresolution. Accordingly, the miniaturization of the semiconductordisplay device tends to be prevented. Therefore, it has been consideredthat there is a limitation in a method of mounting IC formed by a singlecrystalline silicon wafer on a glass substrate. Hence, a technique offorming integrally an integrated circuit including a drive circuit overone substrate with a pixel portion, that is, so-called system on panel,has been attracted attention.

A thin film transistor formed by a polycrystalline semiconductor film(polycrystalline TFT) has advantages that the mobility thereof is twoorders of magnitude higher than that of a TFT formed by an amorphoussemiconductor film, and a pixel portion and a drive circuit around theperiphery thereof of a semiconductor display device can be integrallyformed over one substrate. However, there are problems that the processbecomes complicated due to crystallization of a semiconductor film,yields becomes reduced, and the cost becomes increased compared to thecase of using an amorphous semiconductor film.

In the case of performing laser annealing that is used generally forforming a polycrystalline semiconductor film, energy density requiredfor improving crystallinity should be secured. Accordingly, throughputin the process of crystallization is declined and the crystallinity bythe edge-neighborhood of a laser beam is varied due to that thelongitudinal length of the laser beam has limitations, and so the sizeof a substrate has limitations. The variation of the energy of laserlight causes the variations of the crystallinity of a semiconductorfilm. There is a problem that it is difficult to laser anneal uniformlya subject.

However, a TFT in which a channel formation region is formed by anamorphous semiconductor film can obtain electric field effect mobilityonly of approximately from 0.4 to 0.8 cm²/Vsec. Therefore, the TFT canbe used as a switching element in a pixel portion, however, the TFT isunsuitable for a drive circuit required for high speed operation such asa scanning line drive circuit for selecting pixels, or a signal linedrive circuit for supplying a video signal to the selected pixel.

Especially, in the case of an active matrix light-emitting device amongsemiconductor devices, at least two transistors, that is, a transistorserving as a switching element for controlling the input of a videosignal and a transistor for controlling the supply of current to thelight-emitting element are provided in a pixel. The transistor forcontrolling the supply of current to the light-emitting element ispreferably to have higher ON current than that of the transistor used asa switching element. Therefore, the further improvement of the mobilityof a TFT in a pixel portion is an important challenge for alight-emitting device.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention torealize system on panel without complicating the process of TFT, andprovide a light-emitting device that can be formed by lower cost thanthat of the conventional light-emitting device.

According to the invention, a thin film transistor (TFT) is formed byusing a semiamorphous semiconductor film having an amorphoussemiconductor film in which crystal grains are dispersed, and alight-emitting device is manufactured by using the TFT for a pixelportion or a drive circuit. Since the TFT formed by the semiamorphoussemiconductor film has mobility of from 2 to 10 cm²/Vsec that is 2 to 20times higher than that of a TFT formed by an amorphous semiconductorfilm, a part of or all of the drive circuit can be integrally formedover one substrate with a pixel portion.

Contrary to a polycrystalline semiconductor film, a semiamorphoussemiconductor film (microcrystalline semiconductor film) can bedeposited over a substrate. Specifically, SiH₄ is diluted by 2 to 1000times, preferably, 10 to 100 times in flow ratio and deposited by plasmaCVD. A semiamorphous semiconductor film manufactured by the foregoingmethod includes a microcrystalline semiconductor film having anamorphous semiconductor film dispersed with crystal grains of from 0.5to 20 nm. Therefore, the process of crystallization is not requiredafter forming a semiconductor film contrary to the case of using apolycrystalline semiconductor film. In addition, there is hardly alimitation of a substrate size that is caused by the limitations of thelongitudinal length of a laser beam when crystallization is carried outby laser light. The number of the process for manufacturing a TFT can bereduced, and so yields of a light-emitting device can be improved andthe cost can be reduced.

According to the invention, a semiamorphous semiconductor film may beused for at least a channel formation region. In addition, the channelformation region is not required to be entirely formed in the thicknessdirection by semiamorphous semiconductor, and at least a part of thechannel formation region may be formed by semiamorphous semiconductor.

A light-emitting device comprises a panel sealed with a light-emittingelement and a module provided with the panel mounted with IC or the likeincluding a controller. The invention relates to a device substrate thatcorresponds to one embodiment in which a light-emitting element is notformed in the process of manufacturing the light-emitting device. Aplurality of pixels of the device substrate have a means for supplyingcurrent to the light-emitting element, respectively. The devicesubstrate may be any state such as the state that only a pixel electrodeof the light-emitting device is provided, or the state that a conductivefilm is formed as a pixel electrode and the pixel electrode is notpatterned to be formed.

An OLED (Organic Light Emitting Diode) that is one of light-emittingelements comprises a layer containing an electroluminescent material(hereinafter, electroluminescent layer) generating luminescence uponapplying current (electroluminescence), an anode layer, and a cathodelayer. The electroluminescent layer is formed to be interposed betweenan anode and a cathode, and is formed by a single layer or a laminationlayer. Specifically, the electroluminescent layer comprises a holeinjecting layer, a hole transporting layer, a light-emitting layer, anelectron injecting layer, an electron transporting layer, and the like.Inorganic compounds may be contained in layers composing theelectroluminescent layer. Luminescence in the electroluminescent layeroccurs from the singlet excited state back down to the ground state(fluorescence) and the triplet excited state back down to the singletground state (phosphorescence).

According to the invention, the process of crystallization of adeposited semiconductor film can be omitted, and system on panel of alight-emitting device can be realized without complicating the processof a TFT.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading of the followingdetailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view of a light-emitting device according tothe present invention;

FIGS. 2A and 2B are a circuit diagram and a cross-sectional view of alight-emitting device according to the present invention, respectively;

FIG. 3 is a cross-sectional view of a light-emitting device according tothe invention;

FIG. 4 shows an embodiment of a device substrate according to theinvention;

FIGS. 5A and 5B show embodiments of a device substrate according to theinvention;

FIGS. 6A and 6B are block diagrams for showing a structure of alight-emitting device according to the invention;

FIGS. 7A to 7C are views for showing a process for manufacturing alight-emitting device according to the invention;

FIGS. 8A to 8C are views for showing a process for manufacturing alight-emitting device according to the invention;

FIGS. 9A to 9C are views for showing a process for manufacturing alight-emitting device according to the invention;

FIGS. 10A and 10B are views for showing a process for manufacturing alight-emitting device according to the invention;

FIGS. 11A to 11F are cross-sectional views for showing a pixel includedin a light-emitting device according to the invention;

FIGS. 12A to 12E are circuit diagrams for showing a pixel included in alight-emitting device according to the invention;

FIGS. 13A and 13B show an embodiment of a semiamorphous TFT used for alight-emitting device according to the invention;

FIGS. 14A and 14B show an embodiment of a shift register used for alight-emitting device according to the invention;

FIGS. 15A and 15B are a top view and a cross-sectional view for alight-emitting device according to the invention, respectively; and

FIGS. 16A to 16C show electric appliances using a light-emitting deviceaccording to the invention.

DESCRIPTION OF THE INVENTION

A structure of a TFT used in a light-emitting device according to thepresent invention will be explained hereinafter. FIG. 1 is across-sectional view of a TFT used for a drive circuit and a TFT usedfor a pixel portion. Reference numeral 101 denotes a cross-sectionalview of a TFT used for a drive circuit; 102, a cross-sectional view of aTFT used for a pixel portion; and 103, a cross-sectional view of alight-emitting element supplied with a current by the TFT 102. Both ofthe TFTs 101, 102 are inversed-staggered type (bottom gate type) TFTs.An n-type semiamorphous TFT is more suitable for a drive circuit than ap-type semiamorphous TFT by the reason that the n-type semiamorphous TFThas higher mobility than that of the p-type semiamorphous TFT. In theinvention, a TFT may be either n-type or p-type. In either polarity ofTFT, TFTs formed over one substrate are preferably formed to have thesame polarity for reducing the number of processes.

The TFT 101 of the drive circuit includes a gate electrode 110 formedover a substrate 100, a gate insulating film 111 covering the gateelectrode 110, and a first semiconductor film 112 formed by asemiamorphous semiconductor film, which overlaps with the gate electrode110 via the gate insulating film 111. Further, the TFT 101 includes apair of second semiconductor films 113 serving as a source region or adrain region, and a pair of third semiconductor films 114 formed betweenthe first semiconductor film 112 and the second semiconductor films 113.

FIG. 1 shows the structure in which the gate insulating film 111 isformed by two layers of an insulating film; however, the invention isnot limited thereto. The gate insulating film 111 may be formed by asingle layer or three or more layers of an insulating film.

The second semiconductor films 113 are formed by an amorphoussemiconductor film or a semiamorphous semiconductor film. Impuritiesimparting one conductivity type are doped to the second semiconductorfilms 113. A pair of the second semiconductor films 113 is faced eachother via a region for a channel of the first semiconductor film 112.

The third semiconductor films 114 are formed by an amorphoussemiconductor film or a semiamorphous semiconductor film. The thirdsemiconductor films 114 have the same conductivity type as that of thesecond semiconductor films 113. The third semiconductor films 114 haveproperties of lower conductivity than that of the second semiconductorfilms 113. The third semiconductor films 114 which serve as an LDDregion can relieve electric field concentration at the edge of thesecond semiconductor films 113 which serve as a drain region to preventhot carrier effects. However, the third semiconductor films 114 are notalways necessarily formed. In case of forming the third semiconductorfilms 114, pressure resistance and reliability of TFTs can be improved.Further, in case that the TFT 101 is n-type, n-type conductivity can beobtained without adding impurities imparting n-type when the thirdsemiconductor films 114 are formed. Therefore, in case that the TFT 101is n-type, n-type impurities are not necessarily doped to the thirdsemiconductor films 114. However, impurities imparting p-type are dopedto the first semiconductor film provided with a channel to control theconductivity in order to be close to I-type as much as possible.

A wiring 115 is formed on a pair of the second semiconductor films 113.

The TFT 102 of the drive circuit includes a gate electrode 120 formedover a substrate 100, a gate insulating film 111 covering the gateelectrode 120, and a first semiconductor film 122 formed by asemiamorphous semiconductor film, which overlaps with the gate electrode120 via the gate insulating film 111. Further, the TFT 102 includes apair of second semiconductor films 123 serving as a source region or adrain region, and a pair of third semiconductor films 124 formed betweenthe first semiconductor film 122 and the second semiconductor films 123.

The second semiconductor films 123 are formed by an amorphoussemiconductor film or a semiamorphous semiconductor film. Impuritiesimparting one conductivity type are doped to the second semiconductorfilms 123. A pair of the second semiconductor films 123 is faced eachother via a region to be provided with a channel of the firstsemiconductor film 122.

The third semiconductor films 124 are formed by an amorphoussemiconductor film or a semiamorphous semiconductor film. The thirdsemiconductor films 124 have the same conductivity type as that of thesecond semiconductor films 123. The third semiconductor films 124 haveproperties of lower conductivity than that of the second semiconductorfilms 123. The third semiconductor films 124 which serve as an LDDregion can relieve electric field concentration at the edge of thesecond semiconductor films 123 which serve as a drain region forpreventing hot carrier effects. However, the third semiconductor films124 are not always necessarily formed. In case of forming the thirdsemiconductor films 124, pressure resistance and reliability of TFTs canbe improved. Further, in case that the TFT 102 is n-type, n-typeconductivity can be obtained without adding impurities imparting n-typewhen the third semiconductor films 124 are formed. Therefore, in casethat the TFT 102 is n-type, n-type impurities are not necessarily dopedto the third semiconductor films 124. However, impurities impartingp-type are doped to the first semiconductor film provided with a channelto control the conductivity in order to be close to I-type as much aspossible.

A wiring 125 is formed on a pair of the second semiconductor films 123.

A first passivation film 140 and a second passivation film 141, each ofwhich is formed by an insulating film, are formed so as to cover theTFTs 101, 102, and the wirings 115, 125. The passivation film coveringthe TFTs 101, 102 is not limited to two layers; it may be a single layeror a lamination layer having three or more layers. For example, thefirst passivation film 140 can be formed by silicon nitride, and thesecond passivation film 141 can be formed by silicon oxide. By formingthe passivation film by silicon nitride or silicon oxynitride, the TFT101, 102 can be prevented from deteriorating due to moisture or oxygen.

Either edge of the wiring 125 is connected to a pixel electrode 130 ofthe light-emitting element 103. An electroluminescent layer 131 isformed on the pixel electrode 130. An opposing electrode 132 is formedon the electroluminescent layer 131. In addition, the light-emittingelement 103 has an anode and a cathode. Either electrode serves as apixel electrode and another electrode serves as an opposing electrode.

According to the present invention, the first semiconductor filmincluding a channel formation region is formed by a semiamorphoussemiconductor film so that a TFT having higher mobility than that of aTFT formed by amorphous semiconductor film can be obtained. Therefore, adrive circuit and a pixel portion can be formed over one substrate.

Then, the structure of a pixel included in a light-emitting deviceaccording to the invention is explained hereinafter. FIG. 2A is adiagram of one embodiment of a circuit in a pixel. FIG. 2B is across-sectional view of one embodiment of the pixel corresponding tothat shown in FIG. 2A.

Reference numeral 201 in FIGS. 2A and 2B denotes a switching TFT forcontrolling the input of a video signal to a pixel. Reference numeral202 denotes a drive TFT for controlling the supply of current to alight-emitting element 203. Specifically, drain current of the drive TFT202 is controlled depending on an electric potential of a video signalinput to a pixel via the switching TFT 201, and the drain current issupplied to the light-emitting element 203. In addition, referencenumeral 204 denotes a capacitance element for holding voltage between agate and a source (hereinafter, gate voltage) when the switching TFT 201is turned OFF. The capacitance element 204 is not necessarily provided.

Specifically, the gate electrode of the switching TFT 201 is connectedto a scanning line G. Either the source region or the drain region ofthe switching TFT 201 is connected to a signal line S, and another isconnected to the gate of the drive TFT 202. Either the source region orthe drain region of the drive TFT 202 is connected to a power sourceline V, and another is connected to a pixel electrode 205 of thelight-emitting element 203. Either two electrodes of the capacitanceelement 204 is connected to the gate electrode of the drive TFT 202, andanother is connected to the power source line V.

FIGS. 2A and 2B show a multi-gate structure in which a firstsemiconductor film is shared by a plurality of TFTs connected with theswitching TFT 201 in series and the gate electrode. According to themulti-gate structure, OFF current of the switching TFT 201 can bereduced. Specifically, FIGS. 2A and 2B shows the switching TFT 201having the structure that two TFTs are connected in series; however, amulti-gate structure may be adopted, in which three or more of TFTs areconnected with each other in series, and a gate electrode is connectedthereto. Further, the switching TFT is not necessarily formed to have amulti-gate structure. The switching TFT may be a general single gate TFThaving a gate electrode and a channel formation region.

Then, an embodiment of a TFT included in a light-emitting deviceaccording to the invention different from that shown in FIGS. 1, 2A and2B. FIG. 3 shows a cross-sectional view of a TFT used for a drivecircuit and a cross-sectional view of a TFT used for a pixel portion.Reference 301 denotes a cross-sectional view of a TFT used for a drivecircuit. Reference numeral 302 denotes a cross-sectional view of a TFTused for a pixel portion, and a cross-sectional view of a light-emittingelement 303 supplied with current from the TFT 302.

The TFT 301 in the drive circuit and the TFT 302 in the pixel portioncomprise gate electrodes 310, 320 formed over a substrate 300; a gateinsulating film 311 covering the gate electrodes 310, 320; and firstsemiconductor films 312, 322 formed by a semiamorphous semiconductorfilm, which overlaps with the gate electrodes 310, 320 via the gateinsulating film 311, respectively. Channel protective films 330, 331formed by an insulating film are formed so as to cover the channelformation region of the first semiconductor films 312, 322. The channelprotective films 330, 331 are provided to prevent the channel formationregion of the first semiconductor films 312, 322 from etching during theprocess of manufacturing the TFTs 301, 302. The TFT 301 and TFT 302comprise a pair of second semiconductor films 313, 323 serving as asource region or a drain region; and third semiconductor films 314, 324formed between the first semiconductor films 312, 322 and the secondsemiconductor films 313, 323 respectively.

In FIG. 3, the gate insulating film 311 is formed by two layers ofinsulating films; however, the invention is not limited thereto. Thegate insulating film 311 can be formed by a single layer or three ormore layers of an insulating film.

The second semiconductor films 313, 323 are formed by an amorphoussemiconductor film or a semiamorphous semiconductor film. Impuritiesimparting one conductivity type are doped to the second semiconductorfilms 313, 323. Further, the pair of the second semiconductor films 313,323 face with each other via a region to be provided with a channel.

The third semiconductor films 314, 324, each of which is formed by anamorphous semiconductor film or a semiamorphous semiconductor film, havethe same conductivity type as that of the second semiconductor films313, 323, and have properties of lower conductivity than that of thesecond semiconductor films 313, 323. The third semiconductor films 314,324 serve as an LDD region to relieve an electric field concentrated onthe edge of the second semiconductor films 313, 323 serving as a drainregion. Thus, hot carrier effects can be prevented. The thirdsemiconductor films 314, 324 can enhance the pressure resistance of TFTsto improve reliability thereof; however, the third semiconductor films314, 324 are not necessarily formed. In case that the TFTs 301, 302 aren-type, n-type conductivity can be obtained without doping n-typeimpurities during forming the third semiconductor films 314, 324.Therefore, in case that the TFTs 301, 302 are n-type, n-type impuritiesare not necessarily doped to the third semiconductor films 314, 324.However, impurities imparting p-type are doped to the firstsemiconductor film provided with a channel to control the conductivityin order to be close to I-type as much as possible.

Wirings 315, 325 are formed over the pair of second semiconductor films313, 323, respectively.

A first passivation film 340 and a second passivation film 341, both ofwhich are formed by an insulating film, are formed so as to cover theTFTs 301, 302, and wirings 315, 325. The passivation film covering theTFTs 301, 302 is not limited to two layers. The passivation film can beformed by a single layer, or three or more layers. For example, thefirst passivation film 340 can be formed by silicon nitride, and thesecond passivation film 341 can be formed by silicon oxide. By formingthe passivation film by silicon nitride or silicon oxynitride, the TFTs301, 302 can be prevented from deteriorating due to moisture or oxygen.

Either edge of the wiring 325 is connected to a pixel electrode 370 ofthe light-emitting element 303. An electroluminescent layer 371 isformed on the pixel electrode 370. An opposing electrode 332 is formedon the electroluminescent layer 371. Further, the light-emitting element303 has an anode and a cathode. Either the anode and the cathode is usedas a pixel electrode, and another is used as an opposing electrode.

Then, the structure of a device substrate used for a light-emittingdevice according to the invention.

FIG. 4 shows one embodiment in which only a signal line drive circuit6013 is separately formed, and a device substrate is connected to apixel portion 6012 formed over a substrate 6011. The pixel portion 6012and a scanning line drive circuit 6014 are formed by a semiamorphousTFT. By forming the signal line drive circuit by a transistor havinghigher mobility than that of a semiamorphous TFT, the operation of thesignal line drive circuit that is required to have higher drivefrequency than that of the scanning line drive circuit can bestabilized. Further, the signal line drive circuit may be a transistorformed by a single crystal, a TFT formed by a poly crystal, or atransistor formed by SOI. An electric potential of a power source,various signals, and the like are supplied to the pixel portion 6012,the signal line drive circuit 6013, and the scanning line drive circuit6014, respectively via an FPC 6015.

The signal line drive circuit and the scanning line drive circuit can beformed with a pixel portion over one substrate.

In the case that a drive circuit is formed separately, a substrateprovided with a drive circuit is not necessarily pasted onto a substrateprovided with a pixel portion. For example, the substrate may be pastedonto an FPC. FIG. 5A shows one embodiment in which only a signal linedrive circuit 6023 is separately formed, and a device substrate isconnected to a pixel portion 6022 and a scanning line drive circuit6024, both of which are formed over a substrate 6021. The pixel portion6022 and the scanning line drive circuit 6024 are formed by asemiamorphous TFT. The signal line drive circuit 6023 is connected tothe pixel portion 6022 via an FPC 6025. An electric potential of a powersource, various signals, and the like are supplied to the pixel portion6022, the signal line drive circuit 6023, and the scanning line drivecircuit 6024, respectively via an FPC 6025.

Alternatively, a part of a signal line drive circuit and a part of ascanning line drive circuit may be formed by a semiamorphous TFT overone substrate with a pixel portion. The rest of the signal line drivecircuit and the scanning line drive circuit may be formed separately toconnect electrically to a pixel portion. FIG. 5B shows one embodiment inwhich an analog switch 6033 included in a signal line drive circuit isformed over a substrate 6031 together with a pixel portion 6032 and ascanning line drive circuit 6034, and a shift register 6033 b includedin a signal line drive circuit is separately formed over a differentsubstrate to be pasted onto the substrate 6031. The pixel portion 6032and the scanning line drive circuit 6034 are formed by a semiamorphousTFT. The shift register 6033 b included in the signal line drive circuitis connected to the pixel portion 6032 via an FPC 6035. An electricpotential of a power source, various signals, and the like are suppliedto the pixel portion 6032, the signal line drive circuit, and thescanning line drive circuit, respectively via an FPC 6025.

As shown in FIGS. 4, 5A and 5B, a part of or all of the drive circuit ofa light-emitting device according to the invention can be formed by asemiamorphous TFT over one substrate with a pixel portion.

The method of the connection of the substrate formed separately is notlimited especially; a known COG method, a wire bonding method, a TABmethod, or the like can be applied. The position to be connected withthe substrate is not limited to that illustrated in FIGS. 4, 5A and 5Bin case that electrical connection is possible. Alternatively, acontroller, a CPU, a memory, and the like may be separately formed to beconnected to a substrate.

A signal line drive circuit used in the invention is not limited to anembodiment in which the signal line drive circuit includes only a shiftregister and an analog switch. Besides the shift register and the analogswitch, other circuits such as a buffer, a level shifter, and a sourcefollower may be included. Further, the shift register and the analogswitch are not necessarily formed, another circuit such as a decodercircuit capable of selecting a signal line can be used instead of theshift register, or latch or the like can be used instead of the analogswitch.

FIG. 6A is a block diagram for showing a light-emitting device accordingto the invention. A light-emitting device shown in FIG. 6A comprises apixel portion 701 including a plurality of pixels provided with alight-emitting element; a scanning line drive circuit 702 for selectingeach pixel; and a signal line drive circuit 703 for controlling theinput of a video signal into a selected pixel.

The signal line drive circuit 703 illustrated in FIG. 6A includes ashift register 704 and an analog switch 705. A clock signal (CLK) and astart pulse signal (SP) are input into the shift register 704. Uponinputting the clock signal (CLK) and the start pulse signal (SP), atiming signal is generated in the shift register 704 to be input intothe analog switch 705.

A video signal is fed to the analog switch 705. The analog switch 705samples the video signal depending on the input timing signal to supplythe sampled video signal to a signal line at a subsequent stage.

Then, the structure of a scanning line drive circuit 702 is explained.The scanning line drive circuit 702 includes a shift register 706 and abuffer 707. The scanning line drive circuit 702 may include a levelshifter in some instances. Upon inputting a clock signal (CLK) and astart pulse signal (SP) to the shift register 706, a selecting signal isgenerated in the scanning line drive circuit 702. The generatedselecting signal is buffered and amplified in the buffer 707 to besupplied to a corresponding scanning line. The scanning line isconnected with the gate of a transistor of a pixel per one line. Inorder to turn the transistor of a pixel per one line ON simultaneously,a buffer capable of flowing a large amount of current is used as thebuffer 707.

In the case that a video signal corresponding to R (red), G (green), B(blue) is sampled to be supplied to a corresponding signal line in afull color light-emitting device, the number of terminals for connectingthe shift register 704 to the analog switch 705 is approximately ⅓ ofthe number of terminals for connecting the analog switch 705 to thesignal line of the pixel portion 701. Therefore, by forming the analogswitch 705 over one substrate with the pixel portion 701, the generationratio of connection inferiors can be reduced, the yields can beimproved, and the number of terminals used for connecting a substrateformed separately can be reduced compared with the case that the analogswitch 705 and the pixel portion 701 are respectively formed overseparate substrates.

FIG. 6B is a block diagram for showing a light-emitting device accordingto the invention, which is different from that shown in FIG. 6A. Asignal line drive circuit 713 illustrated in FIG. 6B includes a shiftregister 714, a latch A 715, and a latch B 716. The scanning line drivecircuit 712 has the same structure as that shown in FIG. 6A.

A clock signal (CLK) and a start pulse (SP) are input to the shiftregister 714. Upon inputting the clock signal (CLK) and the start pulse(SP), a timing signal is generated in the shift register 714 to besupplied to the latch A 715 at the first stage. Upon inputting thetiming signal to the latch A 715, a video signal is sequentially writtenin the latch A 715 in synchronization with the timing signal to be held.Further, the video signal is sequentially written in the larch A 715 inFIG. 6B; however, the invention is not limited to this instance.So-called divisional drive can be performed, that is, the latch A 715 ata plurality of stages is partitioned in some groups to input a videosignal to each group in parallel with each other. In addition, thenumber of group is referred to as the number of partition. For example,the case that the latch is partitioned into some groups per four stagesis referred to as partition drive by four-partition.

The time required for completing the write of a video signal to a latchat all stages is referred to as a line period. Practically, the lineperiod may be added with a horizontal retrace period.

Upon completing one line period, a latch signal is supplied to the latchB 716 at a second state, and a video signal held in the latch A 715 iswritten in synchronization with the latch signal to be held in the latchB 716. In the latch A 715 which completes the send of a video signal tothe latch B 716, a next video signal is sequentially written insynchronization with the timing signal from the shift register 714.During the second one line period, a video signal written and held inthe latch B 716 is input into a signal line.

The structure illustrated in FIGS. 6A and 6B is an embodimentillustrative only of a light-emitting device according to the invention.The structures of the signal line drive circuit and the scanning linedrive circuit are not limited to those shown in FIGS. 6A and 6B.

Next, a specific method for manufacturing a light-emitting deviceaccording to the invention is explained.

As a substrate 10, a material of plastic can be used besides glass,quartz, or the like. Alternatively, a metal material such as stainless,aluminum, or the like coated with an insulating film can be used as thesubstrate 10. A first conductive film 11 is formed for forming a gateelectrode and a gate wiring (scanning line) over the substrate 10. Asthe first conductive film 11, a metal material such as chrome,molybdenum, titanium, tantalum, tungsten, aluminum, or the like or analloy of the metal material is used. (FIG. 7A)

The first conductive film 11 is etched to form gate electrodes 12, 13.The edges of the gate electrodes are preferably formed to have a taperedshape since a first semiconductor film or a wiring layer is formed overthe gate electrodes. In the case that the first conductive film 11 isformed by a material mainly containing aluminum, the surface of thefirst conductive film 11 is preferably insulated by anode oxidizationafter etching process. Further, a wiring connected to the gate electrodecan be formed simultaneously according to the process (not shown). (FIG.7B)

A first insulating film 14 and a second insulating film 15 can serve asa gate insulating film by forming over the gate electrodes 12, 13. Inthis instance, the first insulating film 14 is preferably formed as asilicon oxide film, and the second insulating film 15 is preferablyformed as a silicon nitride film. These insulating films can be formedby grow discharge decomposition or sputtering. Especially, for forming adense insulating film with a small amount of gate leak current at lowtemperature, a rare gas element such as argon is included in a reactiongas to be mixed into the insulating film.

Then, a first semiconductor film 16 is formed over such the first andthe second insulating films. The first semiconductor film 16 is formedby a film containing semiconductor having intermediate structure betweenan amorphous structure or crystalline structure (including singlecrystals and poly crystals). The semiconductor has a stable third statewith respect to free energy, and is crystalline having a short-rangeorder and lattice distortion. The semiconductor can be formed to have agrain diameter of from 0.5 to 20 nm to disperse in amorphoussemiconductor. Further, at least one atomic % or more of hydrogen orhalogen is included in the semiconductor as neutralizer for danglingbonds. Hereinafter, such semiconductor is referred to as semiamorphoussemiconductor (SAS) for the sake of convenience. Further, by mixing arare gas element such as helium, argon, krypton, neon, or the like intothe SAS to enhance the lattice distortion, a favorable SAS having goodstability can be obtained. The SAS is disclosed in U.S. Pat. No.4,409,134, for example. (FIG. 7C)

The SAS can be obtained by a silicide gas subjected to grow dischargedecomposition. As a typical silicide gas, SiH₄ can be used. Othersilicide gas such as Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the likecan be used. The SAS can be formed easily by diluting the silicide gaswith a rare gas element selected from the group consisting of hydrogen,hydrogen and helium, argon, krypton, and neon. The dilution rate ispreferably in the range of from 10 to 1000 times. Of course, a reactionproduct for a film is formed by grow discharge decomposition at areduced pressure in the range of approximately from 0.1 to 133 Pa. Highfrequency current of from 1 to 120 MHz, preferably, from 13 to 60 MHzmay be supplied for forming grow discharge. A temperature for heating asubstrate is preferably at most 300° C., more preferably, from 100 to200° C.

An energy band width may be controlled to be from 1.5 to 2.4 eV, or from0.9 to 1.1 eV by mixing a carbide gas such as CH₄ or C₂H₆, or agermanium gas such as GeH₄ or GeF₄ into the silicide gas.

The SAS shows weak n-type electrical conductivity when impurities arenot doped deliberately in order to control a valency electron.Therefore, it becomes possible that a threshold value can be controlledby doping p-type impurities into the first semiconductor film providedwith a channel formation region for a TFT simultaneously with or afterthe formation of the film. As impurities imparting p-type, boron can betypically used. An impurity gas of from 1 to 1000 ppm such as B₂H₆ orBF₃ may be mixed into a silicide gas. The boron may have a concentrationof from 1×10¹⁴ to 6×10¹⁶ atoms/cm³.

As shown in FIG. 8A, a second semiconductor film 17 is formed. Thesecond semiconductor film 17 is formed deliberately without dopingimpurities for controlling a valency electron, and is formed preferablyby a SAS as in the case with the first semiconductor film 16. The secondsemiconductor film 17 is formed to be interposed between the firstsemiconductor film 16 and a third semiconductor film 18 imparting oneconductivity type for forming a source and a drain, and so the secondsemiconductor film 17 serves as a buffer layer. Therefore, the secondsemiconductor film 17 is not necessarily formed, in case that the thirdsemiconductor film 18 imparting one conductivity type that is the sameas that of the first semiconductor film 16 with a weak n-type electricconductivity. The second semiconductor film 17 can vary stepwiseimpurity concentration in case that impurities imparting p-type aredoped to control a threshold value. Accordingly, the secondsemiconductor film 17 becomes preferable embodiment to improve junction.Hence, a TFT to be formed can serve as a low concentration impurityregion (LDD region) that is formed between a channel formation regionand a source or a drain region.

In case that an n-channel type TFT is formed, the third semiconductorfilm 18 imparting one conductivity type may be added with phosphorous asa typical impurity element, and a silicide gas may be added with animpurity gas such as PH₃. The third semiconductor film 18 imparting oneconductivity type cane be formed by semiconductor such as a SAS,amorphous semiconductor, or fine crystalline semiconductor.

As noted above, the process for forming the first insulating film 14 tothe third semiconductor film 18 imparting one conductivity type can becarried out without exposing to the air. Therefore, the variation of TFTcharacteristics can be reduced since each lamination interface can beformed without being contaminated by contaminated impurity elementssuspended in an atmospheric constituent or an atmosphere.

Then, a mask 19 is formed by a photoresist, and then, the firstsemiconductor film 16, the second semiconductor film 17, and the thirdsemiconductor film 18 imparting one conductivity type are etched to beformed separately in island-like shapes. (FIG. 8B)

Thereafter, a second conductive film 20 is formed to form a wiringconnecting with a source and a drain. The second conductive film 20 isformed by aluminum or a conductive material containing mainly aluminum.The layer formed on a semiconductor film may be formed by a laminationlayer comprising titanium, tantalum, molybdenum, tungsten, copper, ornitrides of the foregoing elements. For example, the second conductivefilm 20 may be formed to have the structure in which the first layer isformed by Ta, and the second layer is formed by W; the first layer isformed by TaN, the second layer is formed by Cu; or the first layer isformed by Ti, the second layer is formed by Al, and the third layer isformed by Ti. Further, AgPdCu alloys may be used to either the firstlayer or the second layer. Alternatively, a three lamination layer canbe formed as the second conductive film 20 comprising W, alloys of Aland Si (Al—Si alloy), and TiN sequentially. Tungsten nitride can be usedinstead of the W, an alloy film of Al and Ti (Al—Ti film) can be usedinstead of the Al—Si film, and Ti can be used instead of the TiN. Anelement of from 0.5 to 5 atom % such as titanium, silicon, scandium,neodymium, copper, or the like may be added. (FIG. 8C)

A mask 21 is formed. The mask 21 is formed by pattern formation to forma wiring connecting a source and a drain. The mask 21 serves as anetching mask to form a channel formation region, source and drainregions and LDD regions by removing the second semiconductor film 17 andthe third semiconductor film 18 imparting one conductivity type.Aluminum or a conductive film containing mainly aluminum may be etchedby using a chloride gas such as BCl₃, Cl₂, or the like. Wirings 23 to 26are formed by the etching treatment. Further, etching treatment forforming a channel formation region is carried out by using a fluoridegas such as SF₆, NF₃, CF₄, or the like. In this case, there is hardlydifference of an etching rate from that of the first semiconductor film16 serving as a base film. Accordingly, the time required for theetching is appropriately controlled. As noted above, the structure of achannel etch type TFT can be formed. (FIG. 9A)

A third insulating film 27 for protecting a channel formation region isformed by a silicon nitride film. The silicon nitride film, which can beformed by sputtering or grow discharge decomposition, is required toprevent contaminated impurities such as organic materials, metallicmaterials, moisture suspended in an atmosphere from penetrating into thechannel formation region. Accordingly, the silicon nitride film isrequired to be a dense film. By using the silicon nitride film as thethird insulating film 27, oxygen concentration can be set at most 5×10¹⁹atoms/cm³, preferably, at most 1×10¹⁹ atoms/cm³ in the firstsemiconductor film 16. A silicon nitride film formed by high frequencysputtering with a silicon target using a sputtering gas of nitrogen anda rare gas element such as argon is promoted to be dense by including arare gas element. A silicon nitride film formed by diluting a silicidegas with an inactive gas such as argon by 100 to 500 times by growdischarge decomposition is preferable since the silicon nitride film canbe formed to be dense at low temperature of at most 100° C. Ifnecessary, a fourth insulating film 28 may be formed by a laminationlayer by a silicon oxide film. The third insulating film 27 and thefourth insulating film 28 serve as a passivation film.

A fifth insulating film 29 which is a planarized film is preferablyformed over the third insulating film 27 and the fourth insulating film28. As the planarized film, organic resin such as acryl, polyimide,polyamide, or the like; or an insulating film containing Si—O bond andSi—CH_(x) bond formed by using siloxane material as a start material ispreferably used. Since these materials are hydroscopic, a sixthinsulating film 30 is preferably formed as a barrier film to preventmoisture from penetrating and discharging. As the sixth insulating film30, the foregoing silicon nitride film may be used. (FIG. 9B)

A pixel electrode 31 is formed after forming a contact hole for thesixth insulating film 30, the fifth insulating film 29, the thirdinsulating film 27, and the fourth insulating film 28. (FIG. 9C)

Thus formed channel etch type TFT can obtain an electric field mobilityof from 2 to 10 cm² Vsec by forming the channel formation region by aSAS. Therefore, the TFT can be used as a switching element for a pixel,and an element for forming a drive circuit at a scanning line (a gateline) side.

As noted above, a switching element for a pixel and a drive circuit at ascanning line side can be formed by one TFT, and a device substrate canbe formed by five masks, a gate electrode formation mask, asemiconductor region formation mask, a wiring formation mask, a contacthole formation mask, and a pixel electrode formation mask.

In FIG. 9C, a cathode is preferably used as the pixel electrode 31 sincea TFT of a pixel is n-type. In case that the TFT of the pixel is p-type,an anode is preferably used. Specifically, a known material having smallwork functions such as Ca, Al, CaF, MgAg, AlLi, or the like can be used.

As shown in 10A, a bank 33 is formed by an organic resin film, aninorganic insulating film, or an organic poly siloxane over the sixthinsulating film 30. The bank 33 has as opening portion where the pixelelectrode 31 is exposed. Then, as shown in FIG. 10B, anelectroluminescent layer 34 is formed on the pixel electrode in theopening portion. The electroluminescent layer 34 may be formed by asingle layer or a lamination layer. In case that the electroluminescentlayer 34 is formed by a lamination layer, the electroluminescent layer34 is formed by stacking sequentially an electron injecting layer, anelectron transporting layer, a light-emitting layer, a hole transportinglayer, and a hole injecting layer over the pixel electrode 31 using acathode.

An opposing electrode 35 using an anode is formed to cover theelectroluminescent layer 34. As the opposing electrode 35, a transparentconductive film formed by mixing 2 to 20% of zinc oxide (ZnO) intoindium oxide can be used, besides ITO, IZO, or ITSO. As the opposingelectrode 35, a titanium nitride film or a titanium film may be usedbesides the foregoing transparent conductive film. For planarization ofthe surface, the opposing electrode 35 may be polished by CMP or wipingby a polyvinyl alcohols porous material. After polishing by CMP, thesurface of the opposing electrode 35 may be subjected to UV irradiationor oxygen plasma treatment. A light-emitting element 36 is formed by theoverlap of the pixel electrode 31, the electroluminescent layer 34, andthe opposing electrode 35.

Practically, after completing the process shown in FIG. 10B, a highhermetic protecting film (a laminate film, a ultraviolet curing resinfilm, or the like) hardly discharging gas or a cover member ispreferably used to package the light-emitting element for avoidingexposure to the outside air.

A method for manufacturing the TFT having the structure shown in FIG. 1is illustrated in FIGS. 7A to 10B. A TFT having the structure shown inFIG. 3 can be simultaneously formed. However, the TFT shown in FIG. 3 isdistinguished from that shown in FIGS. 7A to 10B by the fact that thechannel protective films 330, 331 are formed to overlap with the gateelectrodes 310, 320 over the first semiconductor films 312, 322 formedby a SAS.

In FIGS. 1 and 3, after forming a contact hole for the third insulatingfilm (first passivation film) and the fourth insulating film (secondpassivation film), the pixel electrode is formed, and the bank isformed. As the bank, organic resin such as acryl, polyimide, polyamide,or the like; or an insulating film containing Si—O bond and Si—CH_(x)bond formed by using siloxane material as a start material may be used.More specifically, the bank is preferably formed by a photosensitivematerial to form an opening portion over the pixel electrode. The edgeof the opening portion has preferably an inclined plane with acontiguous radius of curvature.

EXAMPLE 1

A semiamorphous TFT that can be used in the present invention can beeither n-type or p-type. The semiamorphous TFT is preferably n-typesince an n-type semiamorphous TFT has high mobility and is suitable forusing as a pixel of a light-emitting device. In this example, across-sectional structure of a pixel is explained using an example of ann-type drive TFT.

FIG. 11B is a cross-sectional view of a pixel used in the case that adrive TFT 7001 is n-type, and light generated in a light-emittingelement 7002 emits passing through an anode 7005. In FIGS. 11A and 11B,a cathode 7003 of the light-emitting element 7002 and the drive TFT 7001are electrically connected each other. An electroluminescent layer 7004and an anode 7005 are sequentially stacked over the cathode 7003. As thecathode 7003, a known material can be used as long as it is a conductivefilm having small work function and reflects light. For example, Ca, Al,CaF, MgAg, AlLi, or the like is preferably used. The electroluminescentlayer 7004 may be formed by a single layer or a lamination layer. Incase that the electroluminescent layer 7034 is formed by a laminationlayer, the electroluminescent layer 7004 is formed by stackingsequentially an electron injecting layer, an electron transportinglayer, a light-emitting layer, a hole transporting layer, and a holeinjecting layer over the cathode 7003. As the anode 7005, a transparentconductive film that is transparent to light formed by mixing 2 to 20%of zinc oxide (ZnO) into indium oxide can be used, besides ITO, IZO, orITSO.

The light-emitting element 7002 corresponds to the overlap region of thecathode 7003, the electroluminescent layer 7034, and the anode 7005. Inthe pixel shown in FIG. 11B, light generated in the light-emittingelement 7002 emits passing through the anode 7005 as denoted by anoutline arrow.

FIG. 11D is a cross-sectional view of a pixel used in the case that adrive TFT 7011 is n-type, and light generated in a light-emittingelement 7012 emits passing through a cathode 7013. In FIGS. 11C and 11D,the cathode 7013 of the light-emitting element 7012 is formed over atransparent conductive film 7017 connected electrically to the drive TFT7011, and an electroluminescent layer 7014 and an anode 7015 are formedsequentially over the cathode 7013. A light-shielding film 7016 forreflecting or shielding light in order to cover the anode 7015 isformed. As the cathode 7013, a known conductive film can be used as longas it has small work function and reflects light as in the case withFIGS. 11A and 11B. The cathode 7013 is formed to have a thickness thatcan transmit light (preferably, approximately from 5 to 30 nm). Forexample, Al having a thickness of 20 nm can be used as the cathode 7013.The electroluminescent layer 7014 may be formed by a single layer or alamination layer as in the case with FIGS. 11A and 11B. Though the anode7015 is not required to transmit light, the anode can be formed by atransparent conductive film as in the case with FIGS. 11A and 11B. Asthe light-shielding film 7016, metals or the like that reflect light canbe used; however, it is not limited to a metal film. For example, resinor the like added with black pigments can be used.

The light-emitting element 7012 is formed by the overlap of the cathode7013, the electroluminescent layer 7014, and the anode 7015. In thepixel shown in FIG. 11B, light generated in the light-emitting element7012 emits passing through the cathode 7013 as denoted by an outlinearrow.

FIG. 11F is a cross-sectional view of a pixel used in the case that adrive TFT 7021 is n-type, and light generated in a light-emittingelement 7022 emits passing through both an anode 7025 and a cathode7023. In FIGS. 11E and 11F, the cathode 7023 of the light-emittingelement 7022 is formed over a transparent conductive film 7027 connectedelectrically to the drive TFT 7021, and an electroluminescent layer 7024and an anode 7025 are formed sequentially over the cathode 7023. As thecathode 7023, a known material can be used as long as it is a conductivefilm having a small work function as in the case with FIGS. 11A and 11B.The cathode 7023 is formed to have a thickness that can transmit light.For example, Al having a thickness of 20 nm can be used as the cathode7023. The electroluminescent layer 7024 may be formed by a single layeror a lamination layer as in the case with FIGS. 11A and 11B. Though theanode 7025 can be formed by a transparent conductive film as in the casewith FIG. 11B.

The light-emitting element 7022 corresponds to the overlap region of thecathode 7023, the electroluminescent layer 7024, and the anode 7025. Inthe pixel shown in FIG. 11F, light generated in the light-emittingelement 7022 emits passing through both the anode 7025 and the cathode7023 as denoted by an outline arrow.

The structure in which the drive TFT is electrically connected to alight-emitting element is explained in this example. A current controlTFT may be formed between the drive TFT and the light-emitting elementto be connected with them.

In all pixels shown in FIGS. 11A to 11F, a protective film can be formedto cover the light-emitting element. As the protective film, a film thatis hard to penetrate substances such as moisture or oxygen that lead todeterioration of the light-emitting element compared to other insulatingfilms is used. Typically, a DLC film, a carbon nitride film, a siliconnitride film formed by RF sputtering, or the like is preferably used.Alternatively, the protective film can be formed by stacking theforegoing film that is hard to penetrate substances such as moisture oroxygen and a film that is easier to penetrate substances such asmoisture or oxygen compared to the foregoing film.

In FIGS. 11D and 11F, in order to emit light from a cathode, there is amethod of using ITO that has less work function by adding with Li can beused besides a method of thickening a film thickness of the cathode.

A light-emitting device according to the invention shown in FIGS. 11A to11F is illustrative and not restrictive, and can be modified based onthe spirit of techniques according to the invention.

EXAMPLE 2

In this example, an example of variation of a pixel using asemiamorphous TFT included in a light-emitting device according to theinvention is explained.

FIG. 12A shows an embodiment of a pixel according to this example. Apixel shown in FIG. 12A comprising a light-emitting element 901, aswitching TFT 902 used as a switching element for controlling the inputof a video signal to the pixel, a drive TFT 903 for controlling acurrent value flowing through the light-emitting element 901, and acurrent control TFT 904 for determining to supply current or not to thelight-emitting element 901. Moreover, a capacitor element 905 forholding electric potential of a video signal may be provided to thepixel as in Embodiment.

The switching TFT 902, the drive TFT 903, and the current control TFT904, which may be either n-type or p-type, have the same polarity. Thedrive TFT 903 operates in a saturation region and the current controlTFT 904 operates in a linear region.

The length of the drive TFT 903 is longer than width. The length of thecurrent control TFT 904 is the same as or shorter than the width.Preferably, the ratio of length to width of the drive TFT 903 is atleast 5. Accordingly, variation of luminance of the light-emittingelement 901 between pixels due to the difference of characteristics ofthe drive TFT 903 can be reduced. Let the channel length of the driveTFT be L1, let the channel width of the drive TFT be W1, let the channellength of the current control TFT be L2, and let the channel width ofthe current control TFT be W2, if L1/W1:L2/W2=X: 1, Xis preferably atleast 5 and at most 6000. For example, if X=6000, it is preferable thatL1/W1=500 μm/3 μm, L2/W2=3 μm/100 μm.

A gate electrode of the switching TFT 902 is connected to a scanningline G. Either a source or a drain of the switching TFT 902 is connectedto a signal line S, and the other is connected to the gate electrode ofthe current control TFT 904. A gate electrode of the drive TFT 903 isconnected to a second power source line Vb. The drive TFT 903 and thecurrent control TFT 904 are connected to a first power source line Vaand the light-emitting element 901 for supplying current supplied fromthe first power source line Va to the light-emitting element 901 asdrain current of the drive TFT 903 and the current control TFT 904. Inthis example, a source of the current control TFT 904 is connected tothe first power source line Va, and a drain of the drive TFT 903 isconnected to a pixel electrode of the light-emitting element 901.

A source of the drive TFT 903 may be connected to the first power sourceline Va, and a drain of the current control TFT 904 may be connected toa pixel electrode of the light-emitting element 901.

The light-emitting element 901 comprising an anode, a cathode, and anelectroluminescent layer interposed between the anode and the cathode.As shown in FIG. 12A, in the case that the cathode is connected to thedrive TFT 903, the cathode serves as a pixel electrode, and the anodeserves as an opposing electrode. Each the opposing electrode of thelight-emitting element 901 and the first power source line Va has anelectrical potential of difference, so that forward bias current issupplied to the light-emitting element 901. The opposing electrode ofthe light-emitting element 901 is connected to an auxiliary electrode W.

Either two electrodes of the capacitor element 905 is connected to thefirst power source line Va, and the other is connected to a gateelectrode of the current control TFT 904. The capacitor element 905 isprovided to hold an electric potential of difference between electrodesof the capacitor element 905 when the switching TFT 902 is in thenon-select state (OFF state). FIG. 12A shows the structure in which thecapacitor element 905 is provided, but the structure of a pixel shown inFIG. 12A is not restrictive. The capacitor element 905 is notnecessarily provided.

In FIG. 12A, the drive TFT 903 and the current control TFT 904 aren-type, and a drain of the drive TFT 903 is connected to the cathode ofthe light-emitting element 901. On the contrary, in the case that thedrive TFT 903 and the current control TFT 904 are p-type, a source ofthe drive TFT 903 is connected to the anode of the light-emittingelement 901. In this instance, the anode of the light-emitting element901 serves as a pixel electrode and the cathode of the light-emittingelement 901 serves as an opposing electrode.

FIG. 12B is a circuit diagram of a pixel shown in FIG. 12A provided witha TFT (erasing TFT) 906 for turning compellingly OFF the current controlTFT 904. In FIG. 12B, like components are denoted by like numerals as ofFIG. 12A. In order to distinguish a first scanning line from a secondscanning line, the first scanning line is denoted by Ga and the secondscanning line is denoted by Gb. A gate electrode of the erasing TFT 906is connected to the second scanning line Gb, and either a source or adrain of the erasing TFT 906 is connected to a gate electrode of thecurrent control TFT 904, and the other is connected to the first powersource line Va. The erasing TFT 906, which can be either n-type orp-type, has the same polarity as that of another TFT in the pixel.

FIG. 12C is a circuit diagram of a pixel shown in FIG. 12A in which agate electrode of the drive TFT 903 is connected the second scanningline Gb. In FIG. 12C, like components are denoted by like numerals as ofFIG. 12A. As shown in FIG. 12C, light emission from the light-emittingelement 901 can be terminated compellingly by switching electricpotential to be fed to a gate electrode of the drive TFT 903.

FIG. 12D is a circuit diagram of a pixel shown in FIG. 12C provided witha TFT (erasing TFT) 906 for turning compellingly OFF the current controlTFT 904. In FIG. 12D, like components are denoted by like numerals as ofFIGS. 12A to 12D, and FIG. 12C. A gate electrode of the erasing TFT 906is connected to the second scanning line Gb, and either a source or adrain of the erasing TFT 906 is connected to a gate electrode of thecurrent control TFT 904, and the other is connected to the first powersource line V. The erasing TFT 906, which can be either n-type orp-type, has the same polarity as that of another TFT in the pixel.

FIG. 12E shows the structure of a pixel without a current control TFT.In FIG. 12E, reference numeral 911 denotes a light-emitting element;912, a switching TFT; 913, a drive TFT; 915, a capacitor element; and916, an erasing TFT 916. A gate electrode of the switching TFT 912 isconnected to the first scanning line Ga, and either a source or a drainof the switching TFT 912 is connected to the signal line S, the other isconnected to a gate electrode of the drive TFT 913. A source of thedrive TFT 913 is connected to the power source line V, and a drain ofthe drive TFT 913 is connected to a pixel electrode of thelight-emitting element 911. An opposing electrode of the light-emittingelement 911 is connected to the auxiliary electrode W. A gate electrodeof the erasing TFT 916 is connected to the second scanning line Gb, andeither a source or a drain of the erasing TFT 916 is connected to a gateelectrode of the drive TFT 913, and the other is connected to the powersource line V.

The structure of a pixel included in a light-emitting device accordingto the invention is not limited to the structure explained in thisexample.

EXAMPLE 3

In this example, one embodiment of a semiamorphous TFT included in alight-emitting device according to the invention is explained.

FIG. 13A is a top view of a semiamorphous TFT. FIG. 13B is across-sectional view of FIG. 13A taken along line A-A′. Referencenumeral 1301 denotes a gate wiring a part of which serves as a gateelectrode. The gate wiring 1301 overlaps with a first semiconductor film1303 formed by semiamorphous semiconductor via a gate insulating film1302. Second semiconductor films 1304 a, 1304 b are formed on the firstsemiconductor film 1303. Third semiconductor films 1305 a, 1305 bimparting one conductivity type are formed on the second semiconductorfilms 1304 a, 1304 b. Each reference numeral 1306, 1307 denotes wiringformed on the third semiconductor films 1305 a, 1305 b.

In a semiamorphous TFT shown in FIGS. 13A and 13B, a channel length canbe kept constantly by spacing out the interval between the thirdsemiconductor film 1305 a and the third semiconductor film 1305 b.Further, by arranging the third semiconductor film 1305 a so as toenclose the edge of the third semiconductor film 1305 b, theconcentration of electric field can be relieved at a drain region sideof a channel formation region. Moreover, since the ratio of a channelwidth to a channel length can be increased, ON current can be increased.

EXAMPLE 4

In this example, one embodiment of a shift register used semiamorphousTFTs having the same polarity is explained. FIG. 14A shows the structureof a shift register according to this example. The shift register shownin FIG. 14A operates by using a first clock signal CLK, a second clocksignal CLKb, and a start pulse signal SP. Reference numeral 1401 denotesa pulse output circuit. A specific structure of the pulse output circuitis illustrated in FIG. 14B.

The pulse output circuit 1401 comprises TFTs 801 to 806 and a capacitorelement 807. A gate of the TFT 801 is connected to a node 2, a source ofthe TFT 801 is connected to a gate of the TFT 805, and the drain of theTFT 801 is given electric potential Vdd. A gate of the TFT 802 isconnected to a gate of the TFT 806, a drain of the TFT 802 is connectedto the gate of the TFT 805, and a source of the TFT 802 is givenelectrical potential Vss. A gate of the TFT 803 is connected to a node3, a source of the TFT 803 is connected to the gate of the TFT 806, anda drain of the TFT 803 is given electrical potential Vdd. A gate of theTFT 804 is connected to the node 2, a drain of the TFT 804 is connectedto the gate of the TFT 805, and a source of the TFT 804 is givenelectrical potential Vss. The gate of the TFT 805 is connected to eitherelectrode of the capacitor element 807, a drain of the TFT 805 isconnected to a node 1, and a source of the TFT 805 is connected toanother electrode of the capacitor element 807 and a node 4. Further,the TFT 806 is connected to either electrode of the capacitor element807, a drain of the TFT 806 is connected to the node 4, and a source ofthe TFT 806 is given electric potential Vss.

The operation of the pulse output circuit 1401 shown in FIG. 14B isexplained. In case of H level, CLK, CLKb, and SP are Vdd, and in case ofL level, the CLK, the CLKb, and the SP are Vss. For the simplificationof explanation, assume that Vss=0.

When the SP becomes H level, the TFT 801 turns ON. Accordingly, electricpotential of a gate of the TFT 805 is increased. Eventually, the TFT 801turns OFF to be in suspension when electric potential of the gate of theTFT 805 becomes Vdd-Vth (Vth is a threshold value of the TFTs 801 to806). On the contrary, when the SP becomes H level, the TFT 804 turnsON. Accordingly, electric potential of the gate of TFTs 802, 806 isreduced to be Vss eventually, and the TFTs 802, 806 turn OFF. The gateof the TFT 803 is L level at this time, and turns OFF.

Then, the SP becomes L level, and the TFTs 801, 804 turn OFF, then,electric potential of the gate of the TFT 805 is held at Vdd-Vth. Whenvoltage between a gate and a source of the TFT 805 is larger than thethreshold value Vth, the TFT 805 turns ON.

When the CLK given to the node 1 changes from L level to H level, thenode 4, that is, electric potential of the source of the TFT 805 becomesincreased since the TFT 805 turns ON. Further, since capacity couplingis presented between the gate and the source of the TFT 805, electricpotential of the gate of the TFT 805 in suspension is increased again inaccordance with the increase of electric potential of the node 4.Eventually, the electric potential of the gate of the TFT 805 becomeshigher than Vdd+Vth, and the electric potential of the node 4 becomesequal to Vdd. The foregoing operation is performed in the pulse outputcircuit 1401 of the second stage or later, and pulse is outputsequentially.

EXAMPLE 5

In this example, an external view of a panel that is one embodiment of alight-emitting device according to the invention is explained withreference to FIGS. 15A and 15B. FIG. 15A is a top view of the panel inwhich a semiamorphous TFT and a light-emitting element formed over afirst substrate are sealed between the first substrate and a secondsubstrate with sealant. FIG. 15B is a cross-sectional view of FIG. 15Ataken along line A-A′.

Sealant 4005 is provided so as to enclose a pixel portion 4002 and ascanning line drive circuit 4004, each of which is formed over a firstsubstrate 4001. A second substrate 4006 is provided over the pixelportion 4002 and the scanning line drive circuit 4004. Therefore, thepixel portion 4002 and the scanning line drive circuit 4004 are sealedwith filler 4007 by the first substrate 4001, and the sealant 4005, andthe second substrate 4006. A signal line drive circuit 4003 formed by apolysilicon semiconductor film over separately prepared substrate ismounted on the region separated from the region enclosed by the sealant4005 over the first substrate 4001 in the panel. In this example, theexample in which the signal line drive circuit with a TFT formed by apoly crystalline semiconductor film is pasted onto the first substrate4001 is explained; however, the signal line drive circuit with atransistor formed by a single crystalline semiconductor film can bepasted thereto. FIG. 15B shows a TFT 4009 formed by a poly crystallinesemiconductor film included in the signal line drive circuit 4003.

The pixel portion 4002 and the scanning line drive circuit 4004, each ofwhich is formed over the first substrate 4001, have a plurality of TFTs.FIG. 15B exemplifies a TFT 4010 included in the pixel portion 4002. Inthis example, the TFT 4010 is assumed a drive TFT, but the TFT 4010 maybe an erasing TFT or a current control TFT. The TFT 4010 is a TFT usingsemiamorphous semiconductor.

Reference numeral 4011 denotes a light-emitting element. A pixelelectrode of the light-emitting element 4011 is electrically connectedto a drain of the TFT 4010 via a wiring 4017. In this example, anopposing electrode of the light-emitting element 4011 is connected to atransparent conductive film 4012. The structure of the light-emittingelement 4011 is not limited to that explained in Embodiment. Thestructure of the light-emitting element 4011 can be appropriatelymodified in accordance with the direction of coupling light, thepolarity of the TFT 4010, or the like.

Various signals and electric potential (not shown in FIG. 15B) fed tothe separately formed signal line drive circuit 4003, the scanning linedrive circuit 4004 or the pixel portion 4002 are supplied from aconnecting terminal 4016 via lead wirings 4014 and 4015.

In this example, the connecting terminal 4016 is formed by a conductivefilm that is used for forming a pixel electrode included in thelight-emitting element 4011. The lead wiring 4014 is formed by aconductive film that is used for forming a wiring 4017. The lead wiring4015 is formed by a conductive film that is used for forming a gateelectrode included in the TFT 4010.

The connecting terminal 4016 is electrically connected to a terminalincluded in an FPC 4018 via an anisotropic conductive film 4019.

As the first substrate 4001 and the second substrate 4006, glass, metals(typically, stainless), ceramic, or plastic can be used. As the plastic,an FRP (Fiberglass-Reinforced Plastics) plate, a PVF (polyvinylfluoride) film, a mylar film, a polyester film, or an acryl resin filmcan be used. Alternatively, a sheet formed by sandwiching aluminum foilby a PVF film or a mylar film can be used.

The substrate that transmits light generated in the light-emittingelement 4011 should be transparent. In this case, a light-transmittingmaterial such as a glass plate, a plastic plate, a polyester film, or anacryl film is used.

As the filler 4007, ultraviolet curing resin or thermal curing resin canbe used besides an inert gas such as nitrogen or argon. PVC (polyvinylchloride), acryl, polyimide, epoxy resin, silicon resin, PVB (polyvinylbutyral), or EVA (ethylene vinyl acetate) can be used. In this example,nitrogen is used as the filler.

FIGS. 15A and 15B show an example of the structure in which the signalline drive circuit 4003 is separately formed to be mounted on the firstsubstrate 4001. However, this example is not limited thereto. Thescanning line drive circuit can be separately formed to be mounted, oronly a part of the signal line drive circuit or only a part of thescanning line drive circuit can be separately formed to be mounted.

This example can be practiced by being combined with structuresexplained in another example.

EXAMPLE 6

A light-emitting device using a light-emitting element is a selfluminous type. Accordingly, the light-emitting device has highvisibility in bright light and wide viewing angle. Therefore, thelight-emitting device can be used for display portions of variouselectric appliances.

As electronic appliances using a light-emitting device according to thepresent invention, a video camera, a digital camera, a goggle typedisplay (a head mounted display), a navigation system, a soundreproduction device (car audio, audio set, or the like), a laptopcomputer, a game machine, a personal digital assistant (mobile computer,a cellular phone, a portable game machine, an electronic book, or thelike), an image reproduction system provided with a recording medium(specifically, a DVD, or the like), or the like can be nominated.Especially, a wide viewing angle is important for a portable electronicappliance since a screen is often viewed from an oblique direction.Therefore, a light-emitting device is preferably used for the portableelectronic appliance. According to the invention, the process forcrystallization is not required after depositing a semiconductor film.Accordingly, a large panel is comparatively easy to be manufactured.Hence, the invention can be effectively used for an electronic applianceusing a large panel of from 10 to 50 inches. Specific examples of suchelectronic appliances are illustrated in FIGS. 16A to 16C.

FIG. 16A shows a display device composed of a housing 2001; a support2002; a display portion 2003; a speaker unit 2004; a video inputterminal 2005; and the like. The display device can be completed byusing a light-emitting device according to the present invention for thedisplay portion 2003. The light-emitting device is a self luminous type,and so back light is not required. Accordingly, the display portion canbe formed to be thinner than that of a liquid crystal display device.The display device includes a display information device such as for apersonal computer; TV broadcast reception; advertisement; and the like.

FIG. 16B shows a laptop computer composed of a main body 2201; a housing2202; a display portion 2203; a keyboard 2204; an external connectionport 2205; a pointing mouse 2206; and the like. The laptop computer iscompleted by using the light-emitting device according to the inventionas the display portion 2203.

FIG. 16C shows a portable image reproduction device including arecording medium (specifically, a DVD reproduction device) composed of amain body 2401; a housing 2402; a display portion A 2403; anotherdisplay portion B 2404; a recording medium (DVD or the like) readingportion 2405; operation keys 2406; a speaker portion 2407; and the like.The display portion A 2403 is used mainly for displaying imageinformation, while the display portion B 2404 is used mainly fordisplaying character information. The portable image reproduction deviceincluding a recording medium includes a game machine, and the like. Theimage reproduction device according to the invention is completed byusing the light-emitting device according to the invention as thedisplay potion A 2403 and the display portion B 2404.

A portion of the light-emitting device that is emitting light consumespower, and so it is desirable to display information in such a mannerthat the light-emitting portion is as small as possible. Accordingly,when the light-emitting device is used to a display portion which mainlydisplays character information, for example, a display portion of aportable information terminal, more particular, a cellular phone or asound reproduction device, it is desirable to drive the light-emittingdevice so that the character information is formed by a light-emittingportion against a background that is a non-emission portion.

As set forth above, the applicable range of the invention is extremelylarge, and can be applied to various fields' electronic appliances.Electronic appliances explained in this example can be practiced bybeing combined with any structure described in Examples 1 to 4.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdescribed, they should be construed as being included therein.

1. A light-emitting device comprising: a pixel portion; and a drivecircuit for controlling operation of the pixel portion, wherein thepixel portion is provided with a pixel including a light-emittingelement and a first TFT for controlling supply of current to thelight-emitting element, wherein each of a second TFT included in thedrive circuit and the first TFT comprises: a gate electrode over asubstrate; a gate insulating film formed over the gate electrode; afirst semiconductor film comprising semi-amorphous silicon over the gateelectrode with the gate insulating film interposed therebetween; a pairof third semiconductor films comprising semi-amorphous silicon over thefirst semiconductor film; and a pair of second semiconductor films overthe pair of third semiconductor films, respectively, wherein the pair ofthird semiconductor films are non-doped semiconductor films, wherein thepair of third semiconductor films and the pair of second semiconductorfilms have n-type conductive type, and wherein the pair of thirdsemiconductor films has lower conductivity than the pair of secondsemiconductor films.
 2. A light-emitting device according to claim 1,wherein the second TFT and the first TFT are covered by a nitride filmor a silicon nitride oxide film.
 3. A light-emitting device according toclaim 1, wherein the pixel further includes a third TFT for controllinginput of a video signal to the pixel, and the third TFT is formed tohave a multi-gate structure.
 4. A light-emitting device according toclaim 1, wherein the drive circuit includes an analog switch.
 5. Anelectronic appliance having the light-emitting device according to claim1, wherein the electronic appliance is selected from the groupconsisting of a video camera, a digital camera, a goggle type display, anavigation system, a sound reproduction device, a laptop computer, agame machine, a personal digital assistant and an image reproductionsystem.
 6. The light-emitting device according to claim 1, wherein thefirst semiconductor film comprises an impurity imparting p-typeconductive type.
 7. A light-emitting device comprising: a pixel portion;and a drive circuit for controlling operation of the pixel portion,wherein the pixel portion is provided with a pixel including alight-emitting element and a first TFT for controlling supply of currentto the light-emitting element, wherein each of a second TFT included inthe drive circuit and the first TFT comprises: a gate electrode over asubstrate; a gate insulating film formed over the gate electrode; afirst semiconductor film comprising semi-amorphous silicon over the gateelectrode with the gate insulating film interposed therebetween; achannel protective film over the gate electrode with the gate insulatingfilm and the first semiconductor film interposed therebetween; a pair ofthird semiconductor films comprising semi-amorphous silicon over thefirst semiconductor film and the channel protective film; and a pair ofsecond semiconductor films over the pair of third semiconductor films,respectively, wherein the channel protective film is interposed betweenthe pair of third semiconductor films, wherein the pair of thirdsemiconductor films are non-doped semiconductor films, wherein the pairof third semiconductor films and the pair of second semiconductor filmshave n-type conductive type, and wherein the pair of third semiconductorfilms has lower conductivity than the pair of second semiconductorfilms.
 8. A light-emitting device according to claim 7, wherein thesecond TFT and the first TFT are covered by a nitride film or a siliconnitride oxide film.
 9. A light-emitting device according to claim 7,wherein the pixel further includes a third TFT for controlling input ofa video signal to the pixel, and the third TFT is formed to have amulti-gate structure.
 10. A light-emitting device according to claim 7,wherein the drive circuit includes an analog switch.
 11. An electronicappliance having the light-emitting device according to claim 7, whereinthe electronic appliance is selected from the group consisting of avideo camera, a digital camera, a goggle type display, a navigationsystem, a sound reproduction device, a laptop computer, a game machine,a personal digital assistant and an image reproduction system.
 12. Thelight-emitting device according to claim 7, wherein the firstsemiconductor film comprises an impurity imparting p-type conductivetype.
 13. A light-emitting device comprising: a pixel portion includinga plurality of pixels; and a drive circuit for controlling operation ofthe pixel portion, wherein each of the plurality of pixels includes alight-emitting element and a first TFT for controlling supply of currentto the light-emitting element, wherein the drive circuit includes asecond TFT, wherein each of the first TFT and the second TFT comprises:a gate electrode over a substrate; a gate insulating film formed overthe gate electrode; a first semiconductor film comprising semi-amorphoussilicon over the gate electrode with the gate insulating film interposedtherebetween; a pair of third semiconductor films comprisingsemi-amorphous silicon over the first semiconductor film; and a pair ofsecond semiconductor films over the pair of third semiconductor films,respectively, wherein the pair of third semiconductor films arenon-doped semiconductor films, wherein the pair of third semiconductorfilms and the pair of second semiconductor films have n-type conductivetype, and wherein the pair of third semiconductor films has lowerconductivity than the pair of second semiconductor films.
 14. Alight-emitting device according to claim 13, wherein the first TFT andthe second TFT are covered by a nitride film or a silicon nitride oxidefilm.
 15. A light-emitting device according to claim 13, wherein each ofthe plurality of pixels further includes a third TFT for controllinginput of a video signal, and the third TFT is formed to have amulti-gate structure.
 16. A light-emitting device according to claim 13,wherein the drive circuit includes an analog switch.
 17. An electronicappliance having the light-emitting device according to claim 13,wherein the electronic appliance is selected from the group consistingof a video camera, a digital camera, a goggle type display, a navigationsystem, a sound reproduction device, a laptop computer, a game machine,a personal digital assistant and an image reproduction system.
 18. Thelight-emitting device according to claim 13, wherein the firstsemiconductor film comprises an impurity imparting p-type conductivetype.